1. Technical Field
This invention relates to the technology of cladless brazing of metals, and more particularly to the use of sacrificial films that form a eutectic with the metals to be joined and create a joint.
2. Discussion of the Prior Art
The prior art has generally employed (i) deposition of relatively thick braze material into a joint area, (ii) use of metal structural sheets clad with brazing material, or (iii) liquid phase pressure diffusion brazing of precoated members.
Deposition of thick brazing material can be by thermal spraying of powder (such as in U.S. Pat. No. 4,606,495) or by hot liquid dipping in a molten bath of clad metal. A non-uniform thickness of the deposited brazed material will often affect physical properties of the joint, producing interfacial flaws.
Clad brazing sheets, although requiring thinner, more uniform layers of brazing material, (see U.S. Pat. No. 4,901,908) restrict and inhibit the type of shapes that can be assembled and structured because the clad brazing sheets are difficult to form into complex shapes (such difficulty is due to the fact that clad layers are rolled bonded to sheet which has limited formability). The clad sheets also cannot be made to fit certain mating components, such as tubes, because tube fabrication requires a seamless joint which cannot be achieved by rolling sheet into tube. Clad sheets may also more readily contain oxides in the clad layer.
Pressure diffusion bonding may also employ clad sheets or braze coated members to affect the joint. Use of pressure not only limits shapes that can be made because of the constraints of how pressure is to be applied, but since diffusion bonding is needed, the type of material that can be used is restricted. For example, in U.S. Pat. No. 4,890,784, the brazing materials were limited to Al-Ni, Al-Zn, or Al-Cu to carry out dissolution of the aluminum in the structural members to be bonded to promote diffusion of the aluminum into binary alloys of the braze material to create a modified interlayer. This is disadvantageous because the components to be brazed must be pressure bonded together to get an effective braze joint; this is most amenable to sheet products, but only in low volumes.